http2/h2demo: deploy with Go 1.13

Update the base image to Go 1.13. It is the latest Go release and
it has TLS 1.3 on by default.

Remove the explicit setting of GOPROXY to https://proxy.golang.org,
since that value is already the default in Go 1.13.

Change-Id: I0d9b684a7f53d27d21b0f92b300ec64d4d47e1fe
Reviewed-on: https://go-review.googlesource.com/c/net/+/196139
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

http2/h2demo/Dockerfile

@@ -2,11 +2,10 @@
 # Use of this source code is governed by a BSD-style
 # license that can be found in the LICENSE file.
 
-FROM golang:1.12 AS build
+FROM golang:1.13 AS build
 LABEL maintainer "golang-dev@googlegroups.com"
 
 ENV GO111MODULE=on
-ENV GOPROXY=https://proxy.golang.org
 
 RUN mkdir /gocache
 ENV GOCACHE /gocache